Geol Paläeont Mitt Ibk Vol 026-0021-0045

Geol. Paläont. Mitt. Innsbruck, ISSN 0378–6870, Band 26, S. 21–45, 2003

5TH WORKSHOP OF ALPINE GEOLOGICAL STUDIES
FIELD TRIP GUIDE E4
ALPINE METAMORPHISM IN THE SCHNEEBERG COMPLEX
AND NEIGHBOURING UNITS (IMMEDIATE VICINITY OF OBERGURGL)
Jürgen Konzett, Georg Hoinkes & Peter Tropper
With 20 figures

I. Ötztal-Stubai Crystalline Complex
and Schneeberg Complex

evolution and structures, indicating an originally
similar tectonic position with subsequent dissection
by Tertiary updoming of the Engadine Window.

1. Geology
The Ötztal-Stubai Crystalline Complex (ÖSCC) is
part of the Austroalpine nappe system which was
emplaced on the Penninic units during the Alpine
Orogeny (Fig. 1). The ÖSCB shows numerous similarities to the Silvretta Nappe in terms of petrologic

1.1. Boundaries of the ÖSCB
The E border of the ÖSCC towards the Tauern Window is formed by the N-S trending Brenner line
which is a W-dipping detachment fault with an

Fig. 1: Simplified geological map of the ÖSCC complex and adjacent areas [modified after Thöni (1981), taken from Elias (1998)]

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estimated minimum lateral displacement of 15-26
km (Selverstone et al. 1995; Fügenschuh 1995). The
N border of the ÖSCC towards the Northern Calcareous Alps is formed by the Inntal line, a sinistral
strike-slip fault. The W border towards the Engadine
Window is formed by the Engadine line (Trümpy
1977). Movements along this line are strike-slip
with respect to the Periadriatic line or oblique slip
(Schmidt & Haas 1989). The S border is tectonically
more complex and in the SW formed by the Schlinig
thrust along which the ÖSCC has overridden the
Austroalpine Sesvenna nappe. Further to the E the
Schlinig line shows a transition into a km-wide
shear zone and at the SE border of the ÖSCB it is
not possible to define the continuation of the
Schlinig line. A possible continuation, however, is
represented by the Passeier and Jaufen lines (Fig. 1,
see also Elias (1998) for a more comprehensive discussion).
1.2. Lithology and structures
The ÖSCC consists of polymetamorphic basement
units of medium- to high-grade gneisses of pelitic to
psammitic origin with intercalations of micaschists,

quartzites, orthogneisses, amphibolites, eclogites
and rare marbles.
The Schneeberg Complex (SC) represents a remnant of a Paleozoic metasedimentray cover with normal stratigraphic relations and consists of several EW trending synclines overturned to the S with a
strikingly different lithology compared to the adjacent ÖSCC rocks (cf. Frank et al. 1987) The SC rocks
are monometamorphic and deeply folded into the
underlying ÖSCC. The strike of the fold structures
changes in W-E direction from NNE-SSW to ENEWSW. To the north, the main syncline („Schneeberger Hauptmulde“) is characterized by a broad central
zone of garnet-micaschists („monotone Serie“) and

small marginal zones of amphibole-bearing rocks,
quartzites and marbles („bunte Randserie“). To the
SW, two smaller synclines – the Schrottner- and
Seeberspitz-synclines - are folded into micaschist
country-rocks. Along the southern margin of the SC,
a zone of crystalline rocks with striking lithological
similarities to the SC occurs. This zone is called „Laas
Series“ and forms a third syncline (Lodner Syncline)
along the SW border of the SC (Fig. 2). In spite of its
similarities to the SC rocks, the Laas Series has to be
assigned to the ÖSCC s.s. due to its polymetamorphic
evolution (Konzett 1990).

Fig. 2: Simplified sketch of the Schneeberg Complex and surrounding rocks (from Hoinkes 1987)

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Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


Relics of the sedimentary Permomesozoic cover of
the ÖSCC are preserved along the western side of the
Brenner line (“Brenner Mesozoic”) and at the northern margin of the SC (“Telfer Weisse”, “Schneeberger
Weisse”). These parautochthonous cover sequences
show only locally developed tectonic contacts
(Fügenschuh & Rockenschaub 1993).
Rapid exhumation of the ÖSCC during late Cretaceous to early Tertiary let to a thrusting of upper
Austroalpine units (“Blaser Decke”, “Steinacher
Decke”) on top of the Permomesozoic sediments (Fügenschuh et al. 2000) (Fig. 1).
Different tectonic styles are prevalent in the

northern and southern part of the ÖSCC: The northern part is characterized by E-W trending fold axes

and an E-W orientation of intercalated orthogneisses. In the southern part of the ÖSCC, no such preferred orientation can be observed. By contrast, kmscale folds with steeply dipping fold axes (“Schlingentektonik” or vortex tectonics) and a partly intense mm-scale folding are the characteristic structural features that can be found in both the ÖSCC
and SC. The age of the “Schlingentektonik” is assumed to be – at least in part – Eo-Alpine (Schmid &
Haas 1989)
The structural evolution of the ÖSCC is complex
and has been subject of several studies (van Gool et
al. 1987; Fügenschuh 1995; Fügenschuh et al. 2000;
Stöckli 1995). A comparison of structures/deforma-

Table 1: Structural evolution of the southern and eastern parts of the ÖSCC and the southeastern adjacent Austroalpine areas based on
the compiled data of van Gool et al. (1987), Fügenschuh (1995) and Stöckli (1995); *) The age of D3 structures is still a matter of discussion; **) D4 structures of van Gool et al. (1987) encompass Cretaceous as well as Tertiary structures;***) Stöckli (1995) distinguishes
deformation phases in the basement (DAA) and Permotriassic cover (DPT) units which not always correlate internally in time. D1AA also
includes top-to-the W directed movements which correlate to pre-D1 of Fügenschuh (1995) (see also Stöckli 1995); ****) Stöckli
(1995) separates two phases in the cover units which are regarded as continuation of the same progressive process of backfolding and
backthrusting.

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

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tion phases distinguished by these authors is given
in Table 1.
2. Metamorphic evolution of the ÖSCC
At least three metamorphic events can be traced
in the ÖSCC rocks whose characteristics are briefly
described below:
2.1. The Caledonian event
This event is recorded by migmatites developed in

biotite-plagioclase gneisses of the central ÖSCC (e.g.
Winnebach migmatite); P-T conditions of 660-685°C
at ≥ 4 kbar (upper amphibolite/lower granulite facies); age of metamorphism 420 – 460 Ma
(Chowanetz 1991, Schweigl 1995).
2.2. The Variscian event
This event reaches conditions of eclogite- to amphibolite facies in the ÖSCC

• eclogite facies: recorded in metabasic (gabbroic/
basaltic) and meta-ultrabasic rocks of the central
ÖSCC; P-T conditions of eclogite facies: around 27
kbar/730°C; age of metamorphism 360-350 Ma
(Miller & Thöni 1995).
• amphibolite facies: recorded in widespread metapelites by Al2SiO5 assemblages ± staurolite; regional
distribution of the Al2SiO5 modifications: Sillimanite: central ÖSCC; kyanite: northern and southern
ÖSCC; andalusite: small area within sillimanite zone
where all three modifications may occur together
(Fig. 3); conditions of amphibolite-facies: around
650°C and 7 kbar (Hoinkes & Thöni 1993); age of
metamorphism 343-331 Ma (Thöni 1993).
2.3. The Eo-Alpine event
The P-T conditions of this event increase from
sub-greenschist conditions in the NE to epidote-amphibolite/eclogite conditions in the SW. This event
leads to the neoformation of Eo-Alpine assemblages
along NE-SW trending isogrades (e.g. zone early
Alpine chloritoid; Purtscheller 1978). Where EoAlpine conditions are lower-grade than Variscian

Fig. 3: Sketch of distribution of Variscian Al2SiO5 assemblages in metapelites of the ÖSCC (taken from Elias 1998); figure caption as
for Fig. 1.

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Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


conditions, a retrogressive breakdown of Variscan
assemblages (e.g. staurolite ⇒ chloritoid; staurolite
⇒ paragonite + chlorite) can be observed.
Peak metamorphic conditions of the Eo-Alpine
event are reached in the southwestern ÖSCC in an
area comprising parts of the SC and the underlying
basement. Rocks in this area record (epidote-)amphibolite to eclogite facies conditions (Konzett &
Hoinkes 1996, Hoinkes et al. 1991, Habler et al.
2001, Exner et al. 2001). The area of max. Eo-Alpine
P-T conditions is characterized by the neo-formation
of staurolite and kyanite in metapelites. The characteristic assemblage in metapelites is Ga + Bio + Mu
+ Plag + Qz ± Pa ± Sta ± Ky.
• epidote-amphibolite facies: recorded in the
monometamorpic SC rocks; P-T conditions around
600°C and 8-10 kbar
• eclogite facies: recorded in basic eclogites and orthogneisses of the polymetamorpic basement rocks
immediately S of the SC; P-T conditions 500–550°C
and ≥ 11-12 kbar; the T-maximum along the P-T
loop followed by the eclogites and their contryrocks is represented by amphibolite facies conditions with 600-650°C and 5-6 kbar.

The Eo-Alpine metamorphic zoning is truncated
by the Passeier-Jaufen line. S of this line subgreenschist to lower greenschist facies conditions are
recorded by both basement and cover units.
The Eo-Alpine overprint leads to a continuous rejuvenation of Variscian Rb-Sr and K-Ar mica cooling
ages in a regional pattern following the Eo-Alpine
mineral isogrades (Fig. 3): In the northwestern ÖSCC

the very weak Eo-Alpine overprint preserves
Variscian mica ages in the range 200-300 Ma; increasing Eo-Alpine conditions are indicated by a decrease in ages towards SW with a complete reset to
ages in the range 80-100 Ma in the area of maximum Eoapline P-T conditions (Elias 1998 and references therein).
3. Cooling and exhumation of the ÖSCC
Cooling of the ÖSCC after the peak of Eo-Alpine
metamorphism started around 90-100 Ma and
ended some 30 Ma later at near surface temperatures. According to Elias (1998) cooling of the ÖSCC
was uneven with periods of rapid cooling around

Fig. 4: Sketch of mica cooling age patterns in the ÖSCC resulting from Eo-Alpine overprint of Variscian assemblages (Taken from Elias
1998); figure caption as for Fig. 1.

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

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Fig. 5: Cooling paths of a vertical profile in the SW part of the SC reflecting max. possible differences in altitude of around 1500 m; (A)
data of the present peak plain of ca. 3500 m which are interpreted to reveal a minimum age of the Eo-Alpine thermal peak; (B) age
data of the lowest sample altitude in the profile of around 2000 m confirming an Eo-Alpine period of rapid cooling around 80 Ma.
(from Elias 1998)

Fig. 6: Modelled (a) temperature-time and (b) temperature-depth paths for the northeastern ÖSCC using one-dimensional thermal
modeling (from Fügenschuh et al. 2000)

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Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

100, 80, and 6-8 Ma and intermittent periods of insignificant cooling (Fig. 5). For the Eastern part of
the ÖSCC, Fügenschuh (2000) found evidence for
late Cretaceous/early Tertiary exhumation due to
low-angle normal faulting. His results indicate decreasing exhumation rates from around 1 mm/a for
late Cretaceous/early Tertiary to 0.7–0.2 mm/a
(Fig. 6). According to Fügenschuh (2000) the pattern
of Eo-Alpine metamorphic assemblages directly reflects the geometry and kinematics of this late Cretaceous/early Tertiary exhumation due to low-angle
normal faulting.
4. References
Chowanetz, E., 1991. Strukturelle und geologische Argumente für eine altpaläozoische Anatexis im Winnebachmigmatit (Ötztal/Tirol, Österreich). Mitteilungen
der österreichischen Geologie- und Bergbaustudenten
37, 15–35.
Elias, J., 1998. The thermal history of the Ötztal-Stubai
Complex (Tyrol, Austria/Italy) in the light of the lateral
extrusion model. Tübinger Geowissenschaftliche Arbeiten 42, 172 pp.
Exner, U., Fussein, F., Grasemann, B., Habler, G., Linner, M.,
Sölva, H., Thiede, R. & Thöni, M., 2001 Cretaceous
Eclogite-Facies Metamorphism in the Eastern Alps:
New Insights, Data and Correlations from an Interdisciplinary Study. Journal of Conference Abstracts 6,
387.
Frank, W., Hoinkes, G., Purtscheller, F. & Thöni, M., 1987.
The Austroalpine unit west of the Hohe Tauern: The
Ötztal-Stubai Complex as an example for the Eo-Alpine Metamorphic evolution. In: Flügel, H.W. & Faupl, P.
(eds.) Geodynamics of the Eastern Alps, Deuticke, Vienna 1987, pp. 179–225.
Fügenschuh, B., 1995. Thermal and kinematic history of
the Brenner area (Eastern Alps, Tyrol). Unpublished
Ph.D. Thesis, ETH-Zürich, 165 pp.
Fügenschuh, B. & Rockenschaub, M., 1993. Deformations
in the hangingwall of the Brenner-fault zone. Terra
Abstracts 5, p. 165.

Fügenschuh, B., Mancktelow, N.S. & Seward, D., 2000.
Cretaceous to Neogene cooling and exhumation history of the Oetztal-Stubai basement complex, eastern
Alps: A structural and fission track study. Tectonics 19,
905–918.
Habler, G., Linner, M., Thiede, R. & Thöni, M., 2001. EoAlpine Andalusite in the Schneeberg Complex (Eastern Alps, Italy/Austria): Constraining the P-T-t-D

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

Path during Cretaceous Metamorphism. Journal of
Conference Abstracts 6, 340.
Hoinkes, G. & Thöni, M., 1993. Evolution of the ÖtztalStubai, Scarl-Campo and Ulten basement units. In: Von
Raumer, J.F. & Neubauer, J.F. (eds.) Premesozoic geology in the Alps, pp 485–494.
Hoinkes, G., Kostner, A. & Thöni, M., 1991. Petrologic constraints for Eo-Alpine eclogite facies metamorphism in
the Austroalpine Ötztal basement. Mineralogy and Petrology 43, 237–254.
Konzett, J., 1990. Petrologie des zentralen Schneeberger
Zugs und des südlich angrenzenden Kristallins im Bereich der Hohen Kreuzspitze, Passeiertal, Südtirol. Unpublished diploma thesis, 237 pp.
Konzett, J. & Hoinkes, G., 1996. Paragonite-hornblende
assemblages and petrological significance: an example
from the Austroalpine Schneeberg Complex, Southern
Tyrol, Italy. Journal of Metamorphic Geology 14, 85–
101.
Miller , C. & Thöni, M., 1995. origin of eclogites from the
Austroalpine Ötztal basement (Tyrol, Austria): geochemistry and Sm-Nd vs. Rb-Sr isotope systematics. Chemical Geology 122, 199–225.
Schmid, S. & Haas, R., 1989. Transition from near-surface
thrusting to intrabasement decollment, Schlinig thrust,
Eastern Alps. Tectonics 8, 697–718.
Schweigl, J., 1995. Neue geochronologische und isotopengeologische Daten zur voralpidischen Entwicklungsgeschichte im Ötztalkristallin (Ostalpen). Jahrbuch der
geologischen Bundesanstalt 138, 131–149.
Selverstone, J., Axen, G. & Bartley, J., 1995. Kinematic
tests of dynamic models for footwall unroofing during

extension in the Eastern Alps. In: Schmid, S., Froizheim,
N., Heilbronner, R., Stünitz, H. & Frey, M. (eds.) Second
Workshop on Alpine Geology, Basel, pp. 56–58.
Stöckli, D., 1995. Tectonics SW of the Tauern Window
(Mauls area, South Tyrol). Southern continuation of the
Brenner Fault Zone and its interaction with other large
fault structures. Unpublished diploma thesis, ETH
Zürich, 270 pp.
Thöni, M., 1993. Neue Isotopendaten zur voralpidischen
Geschichte des Ötztalkristallins. Arbeitstagung der geologischen Bundesanstalt, 10–112.
Trümpy, R., 1977. The Engadine Line: a sinistrl wrench
fault in the Central Alps. Memoirs of the Geological
Society of China 2, 1–17.
Van Gool, J.A.M., Kemme, M.M.J. & Schreurs, G.M.M.F.,
1987. Structural investigations along an E-W crosssection in the southern Ötztal Alps. Flügel, H.W. &
Faupl, P. (eds.) Geodynamics of the Eastern Alps, Deuticke, Vienna 1987, 214–222.

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Topographic and geological maps
Freytag & Berndt Wanderkarte WKS 8 1: 50.000
Passeier-Timmelsjoch-Jaufen
Österreichische Karte 1 : 50.000: Blatt 174 Timmelsjoch
Carta Geologica d’Italia, Foglio IV, Merano (IIEd., Salerno), 1 : 100.000, (1970)

II. Field Stops
Stop No. 1: garnet-biotite-plagioclase-staurolite
gneisses of the Ötztal-Stubai Crystalline Complex
N of the Schneeberg Complex

rolite according to a reaction (simplified in the
NAMSH system):
staurolite + albite + H2O ⇒ paragonite + chlorite +
quartz
This reaction caused the formation of mica+chlorite+quartz-pseudomorphs after lath-shaped staurolite crystals that may reach several cm in length and
can be found on cleavage planes. P-T-conditions
sufficient for Eo-Alpine staurolite formation, however, were reached within the northernmost part of
the SC. The first Eo-Alpine staurolites that appear in
micaschists just within the SC are rich in Zn with up
to 5.6 wt% ZnO and are stabilized towards lower
temperatures by Zn as evidenced by the negative
correlation between KDFe-Mg ga-bio and XZn in staurolite (Fig. 1) (Hoinkes 1981).

Locality: Timmelsjoch; altitude 2474 m; AustrianItalian border
The rocks outcropping in the immediate vicinity of
the customs building are gneisses of the ÖSCC that
form the northern border of the SC. The gneisses
were affected by both Variscian and Eo-Alpine metamorphism: The Varisician metamorphism reached
conditions of the amphibolite facies which led to the
formation of an assemblage plagioclase + biotite +
muscovite + garnet + quartz ± staurolite ± kyanite.
The subsequent Eo-Alpine event in this area reached
P-T conditions slightly below the stability of staurolite. This caused a retrogressive breakdown of stau-

Stop No. 2: Rocks of the Schneeberg Complex
and the underlying basement of the Ötztal-Stubai
Crystalline Complex – northern border of the
Schneeberg Complex
Locality: Timmelsjoch road on the Italian side;

altitude 2200 m; second 180°-road turn to the right.
Along the roadside the northern border of the
main syncline of the Schneeberg Complex towards
the underlying Ötztal-Stubai-Crystalline Complex is
exposed. At this location the distinction between
both units is unambigous due to the difference in lithologies:
Rocks of the ÖSCC:
– monotonous gneisses with
(≤ 1 mm) garnet and biotite

abundant

small

Rocks of the SC:
– garnet-micaschists with large (0.5 to > 1cm) garnets
– amphibole-bearing rocks with large (≥ 1cm) amphiboles
quartzites

Fig. 1: Plot of Zn-concentration in staurolite vs KD for Fe-Mg
exchange between coexisting garnet and biotite for garnetstaurolite micaschists of the SC (from Hoinkes 1981)

28

The SC rocks exposed belong to the marginal series (“bunte Randserie”) of intercalated garnet-micaschsists, amphibolites, hornblende garbenschists,
marbles and quartzites that delimit the main syncline of the SC. Further downhill, rocks of the central
SC main syncline are encountered that are charac-

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

Fig. 1: Chemical zoning patterns of garnets from the SC (samples PL81, R15) and ÖSCC (sample O1)

terized by rather monotonous garnet-micaschists
(“monotone Serie”).
The SC-ÖSCC boundary is located a few meters
uphill from a quartzite band that can unambigously
be assigned to the SC. The actual boundary lies within several meters of gneisses containing small garnets and cannot unambigously be localized in the
field. A definite criterion for the distinction between
SC and ÖSCC rocks, however, is the zoning pattern of
garnets (Fig. 1).
SC rocks are monometamorphic ⇒ garnets show
continuous bell-shaped zoning patterns
ÖSCC rocks are polymetamorphic ⇒ garnets show
discontinuous chemical zoning

Hoinkes, G., 1981. Mineralreaktionen und Metamorphosebedingungen in Metapeliten des westlichen Schneebergerzuges und des angrenzenden Altkristallins.
Tschermaks Mineralogische und Petrographische Mitteilungen 28, 31–54.

Stop No. 3: Rocks of the Schneeberg Complex –
garnet-micaschists of the
SC main syncline (“monotone Serie”)
Locality: Timmelsjoch road on the Italian side;
hamlet of Saltnuss (1680 m)

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

From Saltnuss towards Timmeljoch garnet-micaschists of the SC main syncline are exposed along
the road. The typical assemblage encountered in
these micaschists is

garnet + biotite + muscovite + paragonite +
plagioclase + quartz ± kyanite ± staurolite
Kyanite formation can be attributed to a reaction
paragonite + quartz = kyanite + albite + H2O
that locally proceeded at temperatures of 570-580°C
due to a reduction of a(H2O) in the fluid phase
and/or a reduction a(albite) in plagioclase. The assemblage paragonite + quartz is generally stable in
SC rocks which indicates that (P)-T conditions for
paragonite-breakdown in the pure system NASH
were not yet reached in this area.
For staurolite formation, textures indicate two
possible reactions (Proyer 1989)
paragonite + garnet + quartz = staurolite + plagioclase + H2O
paragonite + biotite + garnet + quartz = staurolite
+ plagioclase + muscovite + H2O
The formation of abundant staurolite in micaschists within the ÖSCC South of the SC (e.g. Laaser
Serie) is due to a discontinuous reaction
garnet + chlorite + muscovite = staurolite + biotite
+ quartz + H2O
which only proceeds at peak metamorphic conditions
of the Eo-Alpine event (570-580°C) (Hoinkes 1981).
Most recently, Habler et al. (2001) reported andalusite from staurolite-garnet-micaschists of the SC main

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syncline formed during the retrogressive stage of the
Eo-Alpine event at temperatures ≥ 540°C.

Habler, G., Linner, M., Thiede, R. & Thöni, M., 2001. Eo-Alpine Andalusite in the Schneeberg Complex (Eastern

Alps, Italy/Austria): Constraining the P-T-t-D Path during Cretaceous Metamorphism. Journal of Conference
Abstracts 6, 340.
Hoinkes, G., 1981. Mineralreaktionen und Metamorphosebedingungen in Metapeliten des westlichen Schneebergerzuges und des angrenzenden Altkristallins.
Tschermaks Mineralogische und Petrographische Mitteilungen 28, 31–54.
Proyer, A., 1989. Petrologie der Rahmengesteine der Pb-Zn
Lagerstätte Schneeberg, Südtirol. Unpublished Diploma
thesis, Innsbruck, 103 pp.

Stop No. 4: Eclogites and eclogite-amphibolites
of the southern Ötztal-Stubai Crystalline Complex
Locality: outcrop of a few m in size along a forest
track that branches of the main road from St. Leonhard i. Passeier to Meran approximately 3 km S of
St. Martin
Description of eclogite assemblages taken from:
Hoinkes, G., Kostner, A. and Thöni, M. (1991) Petrologic Constraints for Eo-Alpine Eclogite Facies Metamorphism in the Austroalpine Ötztal Basement. Mineralogy and Petrology 43, 237–254.
Summary
Metabasites of the southern Ötztal basement
hitherto mapped as amphibolites, were identified as
eclogites. Primary mineral parageneses are tschermakitic to pargasitic green amphiboles, omphacite
(Jd40), garnet II (Gr20-30Py10), phengite (Si3.5), zoisite,
rutile and quartz. Al-pargasite (ca. 20 wt% Al2O3)
rims between garnet and omphacite are interpreted
as retrograde reaction products.
Retrogression of the eclogitic parageneses reflecting decreasing pressure and increasing temperature
conditions are: Symplectites of diopside and plagioclase after omphacite, Al- and Na-poor green amphiboles, grossularite-poor garnet III surrounding
garnet II partly with atoll textures and symplectites
of biotite and plagioclase replacing phengite. Conti-

30

nuation of retrogression with decreasing temperature conditions is indicated by actinolitic amphiboles
and albite-rims between amphibole II and quartz.
A pre-eclogitic metamorphic stage is only recorded by discontinuous garnet I cores of grossularpoor composition. Minimum pressure and temperature conditions of the eclogite stage derived from
Jd-content of omphacite and the gt-cpx-geothermometer are 1–12 kbar and 500-550°C. Maximum
temperature conditions of the posteclogitic stage
were between 600 and 650°C. The presence of these
eclogitic metabasites as lenticular interlayers within
ortho- and paragneisses indicates high pressure metamorphic conditions within the entire rock-sequence. This interpretation is confirmed by the occurrence of phengite-rich micas in orthogneisses indicating
pressures of approx. 11 kbar. Secondary chemical
changes of these phengites to muscovite-rich compositions again show the decreasing pressure conditions in the southern Ötztal basement after the eclogite stage. The age of the eclogite stage is interpreted as Eo-Alpine due to the following arguments:
The eclogites show concordant, tectonically undisturbed contacts to the encasing orthogneiss-metapelite series. This points to a common history during
the last metamorphic stage.
Continuous readjustment from high to intermediate pressure conditions is observed in both eclogites and the acid country rocks.
Isotopic results from the wider study area exclusively yield Cretaceous mineral ages. Rb-Sr data on
eclogite phengites (texturally clearly correlated with
the high-P stage) and thin whole rock slabs of
layered eclogites are in agreement with a dominant
post-Variscan crystallization history, following a
continuous high-P/low-T to low-P/high-T loop.
Field relations and petrography
At the southeastern corner of the zone of highAlpine metamorphic overprint (Alpine staurolite
zone ASZ), eclogitic rocks are exposed in a number of
outcrops. (Hoinkes & Thöni 1987; Fig. 1). These rocks
form s-parallel layers and lenses within acid gneisses
and micaschists. A within-plate basaltic origin for
equivalent amphibolite occurrences from the wider
study area was postulated by Poli (1989).
Eclogites/amphibolites and enclosing metapelites
show concordant and planar contacts indicating a coherent rock sequence. Eclogites form massive dark lay-

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


Fig. 1. (b) Simplified map of part of the southeastern ASZ in the southern Ötztal basement showing the outcrops of eclogitic rocks (full
black). (c) Sketch of the undisturbed contact of eclogite and country rocks gneisses at Untere Stieralm (Saltaus valley)

ers of dm- to m-thickness within garnet-amphibolites.
A continuous transition from eclogite in the center of
the layers to amphibolite at the layer-margins can be
observed. The eclogites are typically banded on a mmto cm-scale with alternating light green pyroxene-rich
and dark green amphibolite-rich layers. The main foliation is parallel to this compositional layering.
Mineralogy
Eclogites with little retrogressive overprint mainly
show a four phase assemblage
clinopyroxene (omp) + garnet (gt) + green amphibole (amp) + quartz (qu)
Additional primary phases that may be present
in various amounts are phengitic muscovite (mus),
zoisite (zo), rutile (rt) and ilmenite (il). Textures indicating the stable coexistence of this four phase
assemblage are (1) straight grain boundaries and
120° triple grain boundaries in quartz-rich domains
(2) inclusions of clinopyroxene, garnet and amphibole within each other. Three generations of garnets can be distinguished based on composition
and textures:
garnet I: chemically distinct cores within garnet II;
rare
garnet II: major gt population; idioblastic grains with
inclusions of qu, omp, amp, zo, qu
garnet III: atoll garnets as overgrowths on the eclogitic fabric; post eclogite stage

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

Primary eclogitic amphiboles are greenish and
occur as dominant matrix minerals in textural equilibrium with omp and gt II. Bluish amphibole may be
present as reaction rims between garnet II and omphacite or its symplectites and are therefore younger
than gt II, omp and primary amphiboles but older
than the symplectites.
Retrogressive alteration is indicated by the following textures:
(1) extremely fine-grained felty symplectites surrounding omp. The symplectites consist of diopside
+ plagioclase ± amphibole
(2) symplectites of greenish amphibole + plagioclase
around primary green amphiboles
(3) albite-rims along quartz-amphibole grain boundaries
(4) myrmecitic symplectites of biotite + plagioclase
surrounding phengites. Symplectite formation
may lead to total replacement of phengites by coarse biotite + plagioclase intergrowths. These indicate the former presence of an eclogitic assemblage even in case the latter was totally obliterated by retrogression
Mineral chemistry
Clinopyroxene
Primary omphacites are moderately zoned with
Jd41-35 in the cores and Jd35-30 at the rims. Secondary symplectitic clinopyroxenes are diopsiderich with only 2-12 mol% Jd-component.

31


Fig. 2: caption on following page

32

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


Fig. 2: (a) Equilibrium textures between omp, amp and gt II;

(b) omp with amp-inclusions and symplectite rim; bluish Alpargasite is present as rim between gt II and omp/symplectite;
(c) eclogite assemblage omp + amp + zoi + qu as inclusion in gt
II; (d) post-eclogitic atoll garnets (gt III); (e) post-eclogitic amphibole (amp II) in a matrix of cogenetic plagioclase; (f) albite
rims between amp II and qu; (g) fine-grained corona of biotite
+ plagioclase surrounding phengitic muscovite; (h) biotite +
feldspar pseudomorph after phengitic muscovite

Garnet
The various garnet generations have the following chemical characteristics:
garnet I: homogeneous cores in garnet II rich in Fe
and Mg and poor in Ca rel. to garnet II
(Fig. 3)
garnet II: Py10-20Gro20-30Alm+Spe50-60; weak zoning
with increasing Mg and Fe and decreasing
Ca towards the rims (Fig 3)
garnet III: lower in Ca and higher in Fe and Mg rel. to
garnet II (Fig. 3)

Amphiboles
primary greenish amphiboles are rich in Na and
poor in Ca with tschermakitic to pargasitic composition. A core-to-rim zoning towards Na-poor pargasite or actinolite may be present. Likewise, secondary
symplectitic amphiboles are Al-poor with actinolitic
compositions. The bluish amphibole rims between
omp and gt may reach 20 wt% Al2O3 and can be
classified as Al-pargasites. Their formation can be
ascribed to a reaction
omp + gt + H2O = amp + Mg-1Si-1Al2
which proceeds after the climax of the high-P
event but prior to the retrogression of omphacite.
Geothermobarometry

Temperatures
Temperatures calculated from Fe-Mg exchange of
coexisting gt II and omp yield temperatures of 545 ±
15°C and 496 ± 15°C based on the calibrations of
Ellis & Green (1979) and Krogh (1988) respectively.
Pairs of atoll garnet rims and symplectitic diopsides
– not in textural equilibrium – yield temperatures of
around 630°C.
Pressures
Minimum pressures for the eclogite stage based
on the jadeite contents of clinopyroxene in equilibrium with quartz are 11–12 kbar for omp cores and 910 kbars for omp rims. Symplectitic diopsides yield
pressures of 5–6 kbar for the retrogressive event.
These results are in agreement with pressures derived from phengite barometry (Fig. 4).
Isotopic results

Fig. 3: (a) discontinuously zoned garnet with gt I in the core and
overgrowth of gt II; profile length 0.3 mm; numbers are wt%
oxides; (b) discont-inuously zoned garnet with gt II in the core
and asym-metric overgrowth of atoll garnet gt III; profile length
1.0 mm

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

Rb-Sr isochron ages show a rather large scatter
ranging from 143 ± 2 (2s) Ma to 71 ± 2.5 Ma. The
oldest age was derived from the least altered of the
investigated eclogite samples (T1302). Sr and Sm
isotopic signatures 87Sr/86Sr = 0.70534 ± 4, 143Nd/
144Nd = 0.51291 ± 2) from the same sample characterize this rock as part of an old (pre-alpine) continental basalt province that has undergone polymetamorphic evolution. The young age of 71 ± 2.5 Ma
was obtained from a strongly retrogressed eclogite

and is similar to an age of 77 ± 3 Ma obtained from
white mica of a gneiss in the immediate vicinity
(Fig. 5). The latter two ages fit into the Eo-Alpine

33


• Petrologic, isotopic data and field relations favor an
Eo-Alpine age of the eclogites in the southern
ÖSCC and indicate a common history of eclogites
and their host rocks since the time of the first penetrative deformation
• The only relics of pre-alpine metamorphism are
cores of discontinually zoned garnets indicating
temperatures > 600°C for this pre-alpine event.

Fig. 4: Phengite barometry for orthogneisses (T1299 = eclogite
country rock, T1319 and T1371 from ASZ but not related to
eclogites) and eclogites (87A43, 87A160). Temperatures were
derived by gt-bio thermometry in adjacent metapelites. Numbers refer to Si per 11 (O, OH) in the phengite formula. Also
shown are minimum P-conditions for eclogite formation (horizontally hatched field) and the symplectite stage (vertically hatched field)

Fig. 5: Mineral isochrons for eclogite amphibolite and enclosing
gneiss at Untere Stieralm

• The Alpine P-T path of the eclogites is characterized by
– pressure peak: ≥ 12 kbar at temperatures of
500-550°C (eclogite facies)
assemblage of eclogites: garnet + omphacite +
phengite + amphibole
phengites in country rocks: Si = 3.32–3.36 apfu

timing of pressure peak: around 140 Ma
– temperature peak: 600-630°C at pressures of
5–7 kbar (amphibolite facies)
assemblage of eclogites: garnet + symplectite
+ plagioclase + amphibole
phengites in country rocks: 3.13 – 3.16 apfu
timing of temperature peak: around 90 Ma
– retrogressive stage: T ≤ 300°C, P < 5 kbar
(greenschist facies)
assemblage: actinolite (rims around primary
amphibole)
phengites in country rocks: 3.2 apfu
timing of retrogression: around 65 Ma
• The assumption of alpine high pressure conditions
in the southern ÖSCC is consistent with Eo-Alpine
P-T conditions of 8-10 kbar at around 600°C in the
Schneeberg Complex immediately N of the eclogites
References

group of ages measured within the ASZ (Thöni
1988).
Conclusions
• Protoliths of the eclogites of the southern ÖSCC are
old continental (within plate) basalts that underwent polymetamorphic evolution during both prealpine (Variscian) and alpine periods of metamorphism

34

Ellis, D.J. & Green, D.H., 1979. An experimental study of
the effect of Ca upon garnet-clinopyroxene Fe-Mg
exchange equilibria. Contributions to Mineralogy and

Petrology 71, 13–22.
Hoinkes, G. & Thöni, M., 1987. New findings of eclogites
within the eoalpine amphibolite grade area of the Ötztal basement. Terra Cognita 7, 96.
Krogh, E.J., 1988. The garnet-clinopyroxene Fe-Mg geothermometer – a reinterpretation of existing experimental data. Contributions to Mineralogy and Petrology 99, 44–48.

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


Poli, S., (1989a). Pre-hercynian Magmatism in the Eastern
Alps: the origin of metabasites from the Austroalpine
basement. Schweizerische Mineralogische und Petrographische Mitteilungen 69, 407–421.
Thöni, M., 1988. Rb-Sr isotopic resetting in mylonites and
pseudotachylites: implications for the detachment and
thrusting of the Austroalpine Basement nappes in the
Eastern Alps. Jahrbuch der geologischen Bundesanstalt
131/1, 169–201.

Stop No. 5: Paragonite-bearing
amphibolitesand garbenschists of the
central Schneeberg Complex

phacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite
marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in
Schneeberg Complex-rocks is indicative of a transition from epidote-amphibolite-facies to greenschistfacies conditions at a flatter PT-gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar-dating of paragonite yields an age
of 84.5±1 Ma corroborating an Eo-Alpine high pressure metamorphic event within the Austro-alpine
unit west of the Tauern Window.
Sample Petrography

Locality: Seewertal; 750–1000 m E of Seewersee
(2056 m); start of track at an altitude of 2020 m

from Timmelsjoch road direction obere Glanegg Alm
From: Konzett, J. & Hoinkes, G. (1996) Paragonite - hornblende assemblages and their petrologic significance: An
example from the Austroalpine Schneeberg Complex, Southern Tyrol, Italy. Journal of Metamorphic Geology 14, 85–
101.

Summary
Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the
Austroalpine Schneeberg Complex, southern Tyrol.
The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase ± garnet ± muscovite. Equilibrium PT-conditions for this assemblage are 550-600°C and 8-10 kbar estimated from
garnet-amphibole-plagioclase-ilmentite-rutile and
Si-contents of phengitic muscovites. In the vicinity
of amphibole paragonite is replaced by symplectitic
chlorite + plagioclase + margarite ± biotite assemblages. Muscovite in the vicinity of amphibole reacts
to form plagioclase + biotite + margarite symplectites The reaction of white mica + hornblende is the
result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore
it is controlled by the availability of fluid on the retrogressive PT-path. Paragonite + hornblende is a
high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane and
represents restricted PT-conditions just below om-

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

The paragonite + hornblende bearing rocks occur at
a single outcrop as a ca. 3 m thick layer in a sequence
of extremely coarse-grained, amphibole bearing calcmicaschists with garben-textures. Within this layer
coarse-grained massive textures lacking any foliation
predominate. The distinctive appearance of these paragonite amphibolites is due to radiating silvery platelets of paragonite reaching up to 5 mm in diameter.
Some dm-wide layers of a fine-grained variety of this
rock type lacking macroscopic paragonite occur intercalated with the massive paragonite amphibolites.
These fine-grained layers are in part well foliated.
Microscopic examination reveals hornblende, paragonite, quartz and plagioclase to be major constituents

of all paragonite amphibolites, together making up
about ca. 70 vol.% of the rock (Fig. 1c). The most striking microscopic feature is the bowtie shape of the
paragonites and their symplectitic replacement (Fig.
1a, c). In one sample muscovite occurs as additional
white mica forming isolated laths or intergrowths
with paragonite; it can be distinguished from the latter by the lack of radiating textures. Muscovites, too,
show replacement textures (Fig. 1d) which will be described below in more detail. Hornblende forms poikiloblastic prisms up to 10 mm in length. Both paragonite and hornblende contain many inclusions of all
matrix phases except garnet, chlorite, cummingtonite
and talc. Garnet occurs as euhedral crystals up to
5 mm in size and contains the same inclusion suite as
paragonite and hornblende. Ankerite and calcite occur
as a minor constituents. While ankerite forms blocky,
isolated crystals, calcite and associated chlorite replace the amphibole. Actinolite was also observed as optically discontinuous rims around hornblende grains.
Talc is present in one sample as small rims around

35


Table 1: Assemblages and estimated modal compositions of paragonite amphibolites

quartz grains associated with ankerite and fine grained calcite. Ti-phases in the matrix and within the
outer zones of garnets are rutile and ilmenite occurring as intergrowths and as separate grains. Titanite
exclusively occurs as inclusions in hornblende and
garnet, and within garnets it is confined to the centers
where it partly reacts to form ilmenite.
White mica breakdown textures
Paragonite breakdown
Within the matrix, paragonite is surrounded by
very fine-grained symplectitic reaction rims. Paragonite breakdown always occurs in the vicinity of
hornblende, and it is only in a few cases that unaltered grain-boundaries between these two phases can

be observed (Fig. 1b). Rare paragonite inclusions in
hornblende and garnet or grains armoured by quartz
in small quartz pods are unaltered.
Two different product assemblages can be identified:
1) plagioclase + chlorite + margarite
2) plagioclase + biotite + margarite ± chlorite

36

Chlorite and biotite occur as lath-shaped grains
embedded in felty-looking plagioclase that can easily be distinguished from clear matrix plagioclase.
With one exception (SW613) margarite can only be
identified with the microprobe. Modal estimates of
the symplectite phases always indicate plagioclase
>> chlorite/biotite > margarite. Mineral inclusions
in paragonite are not affected by the breakdown
reaction. Clinozoisite „survives“ breakdown of the
paragonite host with no change of its modal amount. Biotite produced by paragonite breakdown
can be distinguished from primary matrix biotite
by its smaller grain size and its occurrence as „beards“ along former paragonite - hornblende grain
boundaries. Within a single thin section all stages
of replacement from almost unaltered paragonite
to a complete pseudomorphism can be observed.
Fig. 2 shows typical textures of paragonite amphibolites.
Muscovite breakdown
Like paragonite, muscovite is surrounded by reaction rims. The breakdown assemblage is
plagioclase + biotite + margarite

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

Fig. 1: Photomicrographs of typical textures of pargonite amphibolites (a) bow-tie paragonite showing symplectites along contacts
with hornblende; edge length of image 2.5 mm; (b) unaltered paragonite laths included in hornblende; edge length of image 1.8 mm;
(c) paragonite partly replaced by chlorite + plagioclase + margarite forming a fine-grained symplectite; edge length of image 0.7 mm;
(d) muscovite laths totally replaced by biotite + plagioclase + margarite; edge length of image 0.6 mm; abbreviations: A = plagioclase;
B = biotite; C = chlorite; H = hornblende; M = margarite; P = paragonite; S = symplectite

Textures of muscovite breakdown are different
from those of paragonite breakdown: Plagioclase
forms clear grains and margarite is coarse and can
be identified under the optical microscope. While
paragonite breakdown always commences at the outermost rims leaving the grain-center unaffected
even in an advanced stage of alteration, muscovite
decomposition also occurs in the grain interior along
cleavage plains (Fig. 1d).
Mineral chemistry
Amphiboles
Primary matrix amphiboles and amphibole inclusions in garnet are Mg-hornblendes to tschermakites

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

with Al2O3 contents from 12 to 18 wt%. Most of the
chemical variability can be described in terms of variations in the tschermak (tk) and plagioclase (pl).
(Fig. 2). Amphibole inclusions in garnet show a systematic increase in tk and pl from core to rim Secondary calcic amphiboles (actinolite and actinolitic
hornblende) have low Al2O3 contents between 3 and
7 wt%.
White mica
Paragonite is close to end member composition
with small deviations from 3.00 Si per 11 oxygens. Al
contents are slightly variable and where grain-boundaries are unaltered coexisting paragonite and

hornblende show a correlation in Al(IV)/Al(VI) ratios
as shown in Figure 2. Margarite component of para-

37


Fig. 2: (a) composition of coexisting calcic amphibole and paragonite rims where paragonite share unaltered grain boundaries with
amphibole; (b) plot of wt.% Al2O3 of amphibole vs. radial distance from center of euhedral garnet from sample 601/1.

Fig. 3: (a) Composition of coexisting paragonite and muscovite in sample 610/2 in terms of paragonite and celadonite contents: (b)
compositional profile of celadonite contents for the same muscovite

gonites is between 1.2 and 3.8 mol.%, muscovite
component range from 3.8 to 16.9 mol.% with a maximum in the muscovite bearing sample SW 610/2.
Individual paragonite grains do not show systematic
compositional zoning.
Muscovite is zoned with respect to celadonite
component. On the basis of 11 oxygens and Fetot =
Fe2+, rims have 3.23-3.25 Si pfu, and towards the
core a continuous increase to 3.30–3.32 Si pfu can
be observed along with a decrease in paragonite
component from 12 to 17 mol.%. All analyses show
a small deviation towards trioctahedral composition

38

with 0.04–0.06 octahedral cations pfu. Figure 3
shows the composition of coexisting muscovite and
paragonite.
Margarite formed during paragonite breakdown is

Na-rich and shows 30–49 mol.% paragonite component. Margarite associated with muscovite breakdown has somewhat lower paragonite contents between 24 and 32 mol.%.
Biotite
Primary matrix biotites and secondary biotites
formed during white mica decomposition can be

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


Fig. 4: (a) Composition of primary and secondary biotites: open circles: primary matrix biotites SW610/2; filled circles and triangles:
secondary biotites formed by paragonite and muscovite breakdown SW610/2; open squares: primary matrix biotites SW601/4; filled
squares: secondary biotites formed by paragonite breakdown SW601/4; (b) Al(IV)/Al(VI)-ratios in coexisting amphibole and biotite; filled circles: SW613; open circles SW601/1; filled triangles: SW614; open triangles: SW601/4; filled squares: SW601/3 open squares
SW610/2; crosses: SW162

Fig. 5: (a) compositional zoning profile of an inclusion-rich euhedral
garnet from sample SW601/1; (b) Mn/Mg ratios of coexisting garnet
and amphibole inclusions in euhedral garnet from sample SW601/1 ;
open circles: coexisting garnet and amphibole inclusions; filled
circles: coexisting garnet rims and matrix amphiboles

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

39


distinguished by the higher total Al and partly by a
lower XMg of secondary biotites (Fig. 4a). Primary
biotites and coexisting hornblendes show a positive
correlation in Al(IV)/Al(VI) ratios within individual
samples as well as between different samples
(Fig. 4b).

Garnet
Garnet shows bell-shaped compositional zoning with a
core-to-rim increase in MgO, FeO and Mg/ (Mg+Fe)
ratio and a decrease in MnO (Fig. 5a). CaO either follows
the Mn-trend or, in a few cases, remains nearly con-

stant. Garnet and coexisting amphibole inclusions show
a very regular correlation in their Mn/Mg ratios (Fig. 5b).
and a less pronounced correlation their Fe/Mg ratios.
Plagioclase
Primary matrix plagioclase ranges in composition
from An18 to An30 with a maximum close to An25.
Plagioclase occurring in symplectites around paragonite is enriched in Ca with An35–An55. Muscovite
breakdown produces plagioclase with a compositional range of An24–An50 which overlaps that of matrix plagioclase.

Fig. 6: P-T conditions of metamorphism derived from various geothermobarometers for paragonite amphibolites of the central Schneeberg Complex. GA: garnet-Ca-amphibole thermometer (Graham & Powell 1984); CD H83: results of calcite-dolomite thermometry
(Hoinkes 1983); GB H86: results of garnet-biotite thermometry (Hoinkes 1986)

40

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


Conditions of metamorphism
Temperature
Temperatures can be derived from Fe/Mg exchange
between coexisting amphibole and garnet which was
empirically calibrated by Graham & Powell (1984). A
mean temperature of 562 ± 21˚C was obtained from 7
samples for coexisting garnet rims and amphibole

rims (Fig. 6). Temperatures for amphibole inclusions in
garnet (SW 601/1) are slightly lower with 541 ± 16˚C.
Results are in good agreement with calcite-dolomite
and garnet-biotite thermometry applied to adjacent
tremolite-marbles and staurolite-kyanite-micaschists
by Hoinkes (1983, 1986) with mean values of 566 ±
14 and 581 ± 12˚C respectively (Fig. 6).

An additional constraint on metamorphic pressure
is placed by texturally stable paragonite + clinozoisite + quartz + plagioclase. The relevant equilibrium
amongst these phases is
paragonite + 2 clinozoisite + 2 quartz = 4 anorthite
(1)
+ albite + 2 H2O
The pressure calculated at T = 580˚C and aH2O =
1.0 with representative phase compositions and activities using the TWEEQ-software (Berman et al.
1991) is 10.0 kbar. A reduced water-activity slightly
increases pressures, for example for aH2O = 0.8 the
shift is near 0.5 kbar.
The age of metamorphism

Pressure
Minimum pressures can be derived from celadonite contents of muscovite in sample SW610/2. Muscovite cores from 610/2 have 3.30 - 3.32 Si pfu. corresponding to > 9 kbar at 600˚C according to Massonne & Schreyer (1987).
Coexisting amphibole + garnet + plagioclase
allow application of the empirically calibrated barometers of Kohn & Spear (1990). The resulting pressures are 8.5 ± 0.6 kbar and 9.4 ± 0.5 kbar for Mg- and
Fe-endmember reactions. In addition pressures can
be derived from coexisting garnet + rutile + ilmenite
+ plagioclase (GRIPS) based on the experimental calibration of Bohlen & Liotta (1986). The GRIPSassemblage yields a mean pressure of 9.6 ± 0.6 kbar
from 5 samples.

Previous studies on SC metapelites report Rb/Sr
ages of ca. 90–75 Ma for muscovite and biotite
(Thöni 1983, Thöni & Hoinkes 1987) corresponding
to cooling after the peak of Eo-Alpine metamorphism. Textures and garnet-zoning patterns indicate
a mono-metamorphic history of the SC (Hoinkes
1983). In order to constrain the timing of high Pmetamorphism reported by paragonite + hornblende
assemblages, a paragonite concentrate from sample
SW601/3 was used for 40Ar/39Ar dating. The concentrate displays a mean plateau age of 84.5 ±
1.0 Ma for heating increments from 850–1250˚C
corresponding to ca. 92 % of the released gas (Fig.
7). The heating steps show little variation in apparent K/Ca ratios, indicating gas evolution from com-

Fig. 7: Ar/Ar age of paragonite concentrate from sample SW601/3

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

41


positionally uniform populations of intracrystalline
sites.

paragonite + hornblende → plagioclase + chlorite +
margarite
(2a)
paragonite + hornblende → plagioclase + biotite +
margarite ± chlorite
(2b)

Discussion of phase relations

The high pressure stage
The stable mineral assemblage prior to symplectite-formation was
hornblende + paragonite + clinozoisite + garnet +
biotite ± muscovite + ankerite + plagioclase +
quartz + Ti-phase
This is concluded from both textural and mineral
chemical features: paragonite and hornblende form
inclusions within each other; with the exception of
plagioclase all phases of the high pressure stage
occur as inclusions within garnet. The correlation of
Al(IV)/Al(VI) ratios in coexisting paragonite + hornblende and biotite+ hornblende indicates attainment
of local exchange equilibrium between these phases.
Garnet zoning patterns with continuous decrease
in MnO and increase in XMg from core to rim indicate
garnet growth under increasing PT conditions. This is
supported by systematic changes in amphibole inclusion chemistry and the distribution of Ti-phase inclusions: Amphibole inclusions in garnet show systematically lower values of tschermak and plagioclase
exchange component than matrix amphiboles and a
regular increase in Al2O3 with increasing distance
from garnet core (Fig. 2). These features along with
the Mn/Mg and Fe/Mg correlations between amphibole inclusions and host garnet support the assumption of progressive garnet and amphibole growth
under conditions of local equilibrium. Occasional matrix amphibole zoning with core-to-rim increase in
Al2O3, Na2O, TiO2 and K2O also clearly indicates a
progressive metamorphic evolution. (e.g. Spear 1981)
Figure 8 shows invariant points in the NCMASH-system delimiting the stability-field of tschermakite +
paragonite + H2O. The relative locations of the invariant points were determined with thermodynamic
data of the TWEEQU software using pure endmember
compositions. According to this diagram paragonite +
tschermakite can form during progade metamorphism from the typical greenschist facies assembages
albite + chlorite (against arrow) or albite + chlorite +
clinozoisite (with arrow).

The symplectite stage
Textures indicate two reactions which lead to the
loss of paragonite:

42

Reaction (2a) is dominant, whereas (2b) is well
developed only in two samples 601/4 and 610/2.
Neglecting small amounts of margarite, reaction (2a)
can be idealized as
3 paragonite + 5 tschermakite + 4 H2O = 3 clinochlore + 3 albite + 10 anorthite + quartz (3)
Replacing tschermakite by glaucophane, a reaction equivalent to (3) is
3 paragonite + 5 glaucophane + 4 H2O = 3 clinochlore +13 albite + quartz
(4)
Reaction (4) is well known from blueschists and
marks the transition from blueschist to greenschist
facies conditions during uplift and decompression.
The similarity between reactions (3) and (4) is striking. Both have H2O on the low-entropy side and
thus can only proceed if H2O or in general a fluid
phase is available; both reactions have similar standard state properties. By analogy with reaction (4),
the breakdown of paragonite + hornblende is interpreted as decompression-induced (see also Fig. 8).
The breakdown of phengitic muscovite with formation of plagioclase + biotite + quartz (+ K-feldspar) symplectites is also widespread in high pressure rocks during decompression. According to the thinsection observation, the muscovite breakdown in
SW610/2 can be formulated as
muscovite + Na+ + Ca2+ → biotite + plagioclase +
margarite
(5a)
or
muscovite + amphibole → biotite + plagioclase +
margarite
(5b)

The lack of unaltered muscovite-amphibole grain
boundaries and the presence of an Al-rich phase like
margarite in the symplectite suggest that (5b) was
operative.
Neglecting minor components (e.g. Ti, Fe3+, Mn, K)
the paragonite breakdown which produces plagioclase + chlorite + margarite can be described in the
system SiO2-Al2O3-FeO-MgO-CaO-Na2O-H2O. Calculation of reaction coefficients for reaction (2a) involves a least squares solution of an over-determined system of equations (e.g. Giramitra & Day 1990)
using idealized mineral compositions. For samples

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


SW173/1 and SW169 the following model reactions
are obtained:
173/1 2.32 pa + 1.46 amph + 0.55 H2O → chl +
4.77 plag + 0.33 ma + 0.67 qz
(6)
169
2.51 pa + 1.45 amph + 0.68 H2O → chl +
4.51 plag + 0.63 ma + 0.95 qz
(7)
Reactions (6) and (7) agree with textural observations: H2O is on the reactant side confirming its necessity introduction for the reaction to proceed.
The significance of the paragonite + hornblende
assemblage and regional implications
Evans (1990) calculated a wedge-shaped stability
field for coexisting paragonite + calcic amphibole similar to Fig. 8, denoted as epidote-amphibolite facies, separating greenschist from epidote-blueschist
fields for aluminuous bulk compositions. The paragonite amphibolites presented in this study are considered an example of epidote-amphibolite facies conditions close to the eclogite facies transition in a tectonic setting of crustal thickening created by convergent movement of the African and European continental plates during the early to middle Cretaceous.
The bulk composition of SC paragonite amphibolites is comparable to average oceanic crust compositions especially with respect to Al, Mg and Na, and is
similar to the bulk compositions of the eclogites
(Hoinkes et al. 1991). This indicates that paragonite

+ hornblende stability does not require unusual bulk
compositions but, on the other hand, rather restricted PT conditions: The temperature must be sufficiently high to stabilise Al-rich calcic amphibole instead of sodic or sodic-calcic amphiboles, and the
pressure must be high enough to destabilise plagioclase + chlorite + quartz. In tectonic environments
with steeper PT-gradients (e.g. subduction zones
with PT-paths from greenschist to blueschist to
eclogite facies) the equivalent assemblage would be
glaucophane + paragonite . In the ÖSCC the succession is greenschist to epidote-amphibolite to eclogite facies. There is no evidence (preserved glaucophane inclusions or glaucophane pseudomorphs etc.) for
crossing the blueschist-field during Eo-Alpine metamorphism in any of the rock-types investigated. The
pressure range derived for SC paragonite amphibolites is supported by garbenschist-like assemblages in
the vicinity containing Al-rich calcic amphibole +
muscovite + kyanite ± staurolite. From his experi-

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003

mental work Hoschek (1990) concluded that muscovite + calcic amphibole and especially calcic amphibole + kyanite is favoured by high pressures. Selverstone et al. (1984) report a paragonite + calcic amphibole + kyanite + staurolite assemblage from the
Tauern Window at 10 kbar and 550˚C, they ascribe
its stability to PT conditions usually not attained during regional metamorphism.
Textural observations and derivation of reaction
stoichiometry show that preservation of the paragonite + hornblende assemblage is critically dependent
on the availability of fluid and the degree to which
fluid can penetrate the rock, the latter being controlled by grain size and deformation: the least retrogressed paragonite amphibolites are coarse-grained and show massive textures while the strongly
retrogressed varieties are fine-grained and/or foliated. The survival of high-P assemblages due to limited fluid access is widespread and well documented
in the Western Alps.
The breakdown of paragonite + hornblende, even
if it takes place „irreversibly“, i.e. is overstepped, is
only possible where fluid can access the rock. Thus,
calculation of the PT-conditions of paragonite +
hornblende breakdown assuming equilibrium may be
grossly misleading. A likely fluid source is dehydration in adjacent metapelites or the paragonite + hornblende reaction may be triggered by fluid released
from preceeding muscovite breakdown which, at the

same time, can supply K required for biotite formation in paragonite-symplectites.
Conclusions
• Paragonite + hornblende is an equilibrium assemblage representing rather restricted PT-conditions
within the epidote-amphibolite facies in a mafic
bulk system.
•Paragonite amphibolites are indicators of high pressure regional metamorphism close to, but still
below omphacite stability. They must be considered
direct precursors of low pressure-high temperature
eclogites in a regional metamorphic regime. Pressures derived at a maximum temperature of
~600˚C are 8–10 kbar.
• Ar/Ar-dating of paragonite yields an Eo-Alpine plateau age of 84.5 ± 1.0 Ma indicating a thermal climax
of Eo-Alpine metamorphism around 100–95 Ma.

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Fig. 8: Invariant points in the system NCMASH delimiting the stability field of paragonite
+ tschermakite; bold arrows indicate a suggestes P-t evolution path during Eo-Alpine metamorphism.
Abbreviations: pa = paragonite, tsch = tschermakite, ab = albite, an = anorthite, czoi =
clinozoisite, chl = chlorite, py = pyrope, qz = quartz, w = water

Geol. Paläont. Mitt. Innsbruck, Band 26, 2003


• Breakdown of paragonite + hornblende to form the
greenschist facies assemblage chlorite + plagioclase + margarite ± biotite occurs during uplift and
decompression of the Schneeberg Complex rocks
after the peak of Eo-Alpine metamorphism. It is

considered a high temperature equivalent of paragonite + glaucophane breakdown which indicates
the blueschist-greenschist facies transition in high
pressure regimes. By analogy, paragonite + glaucophane-bearing rocks are precursors of low temperature-high pressure eclogites in subduction zones.
Reaction of phengitic muscovite to plagioclase +
biotite + margarite is ascribed to the same process
but may not have occured contemporaneously.
• The occurrence of high pressure metamorphism in
the Schneeberg Complex is thought to be related to
eclogite formation in the Ötztal-Stubai Crystalline
Basement southeast of the Schneeberg Complex
and strongly supports the assumption of an Eo-Alpine age of these eclogites (Hoinkes et al. 1991).
References
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Thöni, M. & Hoinkes, G., 1987. The Southern Ötztal Basement: Geochronological and Petrological Consequences of Eo-Alpine Metamorphic Overprinting. In: Geodynamics of the Eastern Alps (eds. Flügel, H. W. &
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Authors’ addresses:
Dr. Jürgen Konzett, Dr. Peter Tropper, Institut für Mineralogie und
Petrographie, Universität Innsbruck, Innrain 52, 6020 Innsbruck
Univ.-Prof. Dr. Georg Hoinkes, Institut für Mineralogie und
Petrologie, Universität Graz, Universitätsplatz 2/II, 8010 Graz
e-mail:

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